There is a long history of technological development and innovation in the field of hydrocarbon exploration and extraction. As a capital intensive industry, the hydrocarbon extraction industry has much incentive to optimize and maximize production from particular hydrocarbon-bearing formations. For example, unconventional reservoirs are hydrocarbon reservoirs where permeability is low and stimulation is required for profitable production.
In the production of hydrocarbon from unconventional geologic formations such as shale, one common extraction optimization technique is to stimulate the hydrocarbon reservoir by creating multiple hydraulic fractures along a multi-stage fractured horizontal well. This technique is commonly referred to as “fraccing”. The resulting hydrocarbon production in a fraccing scenario is a result of flow in matrix, in natural fracture networks and in the hydraulic fractures themselves.
There are a number of problems in trying to model hydrocarbon production in a fractured geological formation with high heterogeneity. For example, during multi-stage hydraulic fracturing, some pre-existing natural fractures are reactivated. Hydraulic fractures and the active natural fractures comprise a hydraulically conductive flow network for hydrocarbon production. In other circumstances, unconventional formations along the horizontal well are known to be highly heterogenous in petrophysical and geological characteristics. In this type of the circumstance, the formation reacts differently at different fracturing stages and the generated fracture network along the horizontal well was also highly heterogenous. A modeling method can only be reliable by incorporating consideration of the heterogeneity of these post fracturing unconventional formations.
Innovative fracturing techniques are also being developed and used by many frac companies, including two representative techniques referred to under the SIMULFRAC and ZIPPERFRAC brands. In either the SIMULFRAC or ZIPPERFRAC methods, two or more Parallel horizontal wells are drilled and then perforated and fractured an alternate intervals along the wellbore. This creates a high density network of hydraulic fractures and accordingly, the stimulated volume that each hydraulic fracture can control is relatively reduced. The stimulated volume beyond hydraulic fracture tips also becomes smaller and its inside flow may no longer behave like linear flow. Existing modeling methods are inapplicable, if they assumed that the flow beyond the fracture tips is linear.
Another complication is that fluid flow mechanisms in unconventional reservoirs are quite complex when compared with conventional formations. Darcy's law is always deficient in such reservoirs. Gas diffusion and desorption appear simultaneously in production of some unconventional gas reservoirs. Moreover, high dependence of reservoir permeability on stress is been confirmed by many experiments. Few methods have been developed in any technical literature or approach to comprehensively incorporate all of these complex flow mechanisms into modeling or evaluating the production of unconventional reservoirs.
Another one of the issues associated with the modelling or execution of fracture treatment in unconventional formations is the difficulty associated with forecasting or accurately modelling the likely production from the formation. Although complex analytical and numerical methods may be developed to represent the fluid flow towards a multi-stage fractured horizontal well, these methods require high computing capacity, long computing time, and also show difficulty in iterative applications. One of the main technical reasons for the difficulty in these computations is the low matrix permeability.
Hydrocarbon produced from each fracture stage mainly comes from stimulated reservoir volume around the hydraulic fracture(s), which provides possibilities for decomposing the reservoir into smaller parts. A fast, simple and reliable method of considering the production from an unconventional reservoir, based upon a decomposition of the unconventional reservoir into smaller parts would it is believed be well received.
If it were possible to create a method for the generation of a type curve of hydrocarbon production from an unconventional reservoir which is stimulated by multi-stage hydraulic fractures this would be desirable in the hydrocarbon production industry.